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In the temperature range from 0.65 K to 24.5561 K the ITS-90 is defined by specified vapour-pressure equations for 3He (0.65 K to 3.2 K) and 4He (1.25 K to 5.0 K) and interpolation equations for a constant-volume gas thermometer (ICVGT) using 3He or 4He. The definitions are given in Sections 3.1 and 3.2 of the ITS-90 text [1], and methods for realising the scale in this range are outlined in Chapters 4 and 5 of the Supplementary Information for the ITS-90 [2]. Direct realisations of the ITS-90 by these methods require relatively sophisticated apparatus and time-consuming experiments, and consequently they are rarely carried out. The situation is acceptable in practice only because rhodium-iron resistance thermometers are available which, once calibrated, are able to maintain their calibrations reliably for long periods of time. They are the practical thermometers on which realisations of the ITS-90 are most accurately maintained, disseminated and compared. CCT-K1 is therefore a comparison of calibrated rhodium-iron resistance thermometers. The comparison has been coordinated by NPL, and participation was invited in a circular to CCT representatives in March 1997, see Appendix 1. It was a prerequisite of participatio